Fluid flow separation can occur when a compressible or incompressible fluid flows over a surface, in particular a convex curved surface, such as an interior surface of a fluid conduit or an exterior surface of a body immersed in a fluid. Flow separation can occur under laminar or turbulent flow conditions, depending upon the boundary layer fluid flow characteristics and the geometry of the surface. It is often desirable to inhibit flow separation in order to reduce form drag or in order to increase aerodynamic lift. In general, the farther along a curved surface that a fluid travels before separation, the better the resulting form drag and aerodynamic lift.
In the case of aerodynamic surfaces, the aerodynamic performance or efficiency of a particular surface, for example an airfoil, such as an airplane wing, rotor blade, turbine or compressor blade, windmill, fan or propeller blade, is strongly dependent on the lift force generated by the airfoil. To this end, active flow control (AFC) techniques have been utilized to increase the lift of airfoils by inhibiting or delaying separation of the fluid flow over the aerodynamic surface.
Active flow control techniques include providing ports or openings in the surface of an airfoil, and providing steady air flow into or out from the ports or openings, or unsteady (e.g., alternating) fluid flow into and out from the ports and openings. Active flow control techniques have proven to be effective in increasing the lift coefficient of airfoils, decreasing the drag coefficient, or both, thereby increasing the aerodynamic performance or efficiency of the airfoil.
Active flow control techniques are particularly advantageous under conditions where large flow separation over an aerodynamic surface would otherwise exist. Such conditions are common at airfoil leading-edge slats and trailing-edge flaps during periods during which high lift is generated.
The high lift auxiliary surfaces, such as leading-edge slats or trailing-edge flaps, are required primarily during relatively slow-speed flight, or during take-off and landing. The potential lift performance generally is not reached and a drag penalty generally occurs during the deployment of leading-edge slats or trailing-edge flaps due to the creation of localized flow separation regions. The size of these flow separation regions depends on factors such as the free stream angle of attack, the relative flow velocity of the fluid stream with respect to the aerodynamic surface, the airfoil chord lines, geometry and the deflection angle of the leading-edge slats or the trailing-edge flaps.
By reducing or inhibiting flow separation, a corresponding increase in lift and reduction in drag can be achieved. Active flow control methods can reduce or inhibit flow separation, for example, by introducing relatively high-velocity fluid flow into the fluid stream immediately above the aerodynamic surface in order to increase the kinetic energy of the fluid stream boundary layer, thereby maintaining attachment of the boundary layer farther along the surface. Similarly, removing relatively low-velocity fluid from the flow stream adjacent the aerodynamic surface can result in a net increase of the kinetic energy of the flow stream boundary layer and help to reduce or inhibit flow separation. However, some existing active flow control methods and devices can be prohibitively fragile or heavy, and can have limited power capacity.
Accordingly, it is desirable to provide a method and apparatus that provides active flow control and is robust against physical damage, lightweight, and has a relatively high power capacity.